The human spine comprises individual vertebras 330 (segments) that are connected to each other to form a spinal column 329, shown in FIG. 12A. Referring to FIGS. 12B and 12C, each vertebra 330 has a cylindrical bony body (vertebral body) 332, three winglike projections (two transverse processes 333, 335 and one spinous process 334), left and right facet joints 346, lamina 347, left and right pedicles 348 and a bony arch (neural arch) 336. The bodies of the vertebrae 332 are stacked one on top of the other and form the strong but flexible spinal column. The neural arches 336 are positioned so that the space they enclose forms a tube, i.e., the spinal canal 337. The spinal canal 337 houses and protects the spinal cord and other neural elements. A fluid filled protective membrane, the dura 338, covers the contents of the spinal canal. The spinal column is flexible enough to allow the body to twist and bend, but sturdy enough to support and protect the spinal cord and the other neural elements. The vertebras 330 are separated and cushioned by thin pads of tough, resilient fiber known as inter-vertebral discs 340. Disorders of the spine occur when one or more of the individual vertebras 330 and/or the inter-vertebral discs 340 become abnormal either as a result of disease or injury. In these pathologic circumstances, fusion of adjacent vertebral segments may be tried to restore the function of the spine to normal, achieve stability, protect the neural structures, or to relief the patient of discomfort.
Several spinal fixation systems exist for stabilizing the spine so that bony fusion is achieved. The majority of these fixation systems utilize rods that attach to screws threaded into the vertebral bodies or the pedicles. In some cases plate fixation systems are also used to fuse two adjacent vertebral segments. This construction usually consists of two longitudinal plates that are each placed laterally to connect two adjacent pedicles of the segments to be fused. This system can be extended along the sides of the spine by connecting two adjacent pedicles at a time similar to the concept of a bicycle chain. Current plate fixation systems are basically designed to function in place of rods with the advantage of allowing intersegmental fixation without the need to contour a long rod across multiple segments. Both the plating systems and the rod systems add bulk along the lateral aspect of the spine limits access to the pars and transverse processes for decortication and placement of bone graft. In order to avoid this limitation many surgeons decorticate before placing the rods, thereby increasing the amount of blood loss and making it more difficult to maintain a clear operative field. Placing rods or plates lateral to the spine leaves the center of the spinal canal that contains the dura, spinal cords and nerves completely exposed. In situations where problems develop at the junction above or below the fused segments necessitating additional fusion, the rod fixation system is difficult to extend to higher or lower levels that need to be fused. Although there are connectors and techniques to lengthen the fixation, they tend to be difficult to use and time consuming.
Accordingly, there is a need for a spinal stabilization device that does not add bulk to the lateral aspect of the spine and does not limit access to the pars and transverse processes for decortication and placement of bone graft.